The invention relates to processes for high-temperature catalytic conversion of ammonia in a two-stage catalytic system in which the first stage, in the downstream direction of the flow of gas comprising at least ammonia and oxygen, is a layer of nettings made of alloys comprising platinoids, while the second stage is composed of catalysts that do not comprise noble metals. The field of use of the proposed invention extends to the production of nitric acid and prussic acid, and also hydroxylamine sulfate.
High-temperature catalytic conversion of ammonia is carried out, as a rule, at atmospheric and higher pressures (0.1-1.8 MPa) on nettings woven or knit from platinoid threads (an alloy of platinum with rhodium or platinum with rhodium, palladium and/or other platinoids). The reaction is accompanied by intensive release of heat, takes place at a very high speed and is limited by the processes of mass transfer.
The temperature of the gas during the production of prussic acid at a pressure of 0.2 MPa may reach 1300xc2x0 C.
In the production of nitric acid during the conversion of ammonia into nitric oxide at a pressure of from 0.7 to 1.8 MPa, the optimum temperature of the nettings is 900-940xc2x0 C. and the yield of nitric oxides is about 95%. At atmospheric pressure, the process takes place at 810-870xc2x0 C. with a yield of 97-98%. The remaining ammonia is consumed for the formation of nitrogen. The processes of conversion of ammonia are characterized by low hydrodynamic resistance (several tens of mm of water column) and high linear speeds (to 15 m/s) [R. Pickwell, Nitric Acid Plant Optimization, Chem. and Ind. 4, 21, 1981, 114]. Depending on the conditions under which the process is carried out, the service life of the packet of platinoid nettings is from 1.5 to 16 months.
Substantial losses of platinoids in the course of the industrial process are characteristic for all processes of high-temperature conversion of ammonia, carried out with platinoid nettings.
When catalytic reactions are carried out on platinum, loosening upxe2x80x94corrosion of the surface of the catalyst is observed. The formation of crystalline structures in the course of the reaction of oxidation of ammonia is accompanied by an increase of the surface of the catalyst by up to 20 times (wherein the diameter of the catalytic platinoid thread is also increased), by losses of platinum in the form of volatile platinum oxides (chemical losses) and by mechanical carrying away of particles of the catalyst. At the end of the run, the surface of the first nettings in the downstream direction of the flow of gas is reduced, mainly as a result of loss of platinum. Catalytic corrosion takes place at a different speed depending on the positioning of the platinoid netting in the packet. Accordingly, both the surface of each netting and losses of platinum for each netting change. During the length of the run of the nettings, the loss of platinoids accompanying the corrosion of the surface of the nettings may be up to ⅔ of the initial load. In the process of obtaining prussic acid, wherein the temperature of interaction of the reagents is substantially higher than is characteristic for the production of nitric acid, the catalytic nettings are even more fragile, and a tendency is observed toward the melting of the nettings. The losses of platinum also depend on the physicochemical properties of the alloy.
However, the losses of platinoids depend on the technological parameters and the structural execution of the process to a substantially greater degree than on the composition of the catalysts. in systems under pressure, the process is carried out at a higher temperature. The strength of the catalyst, linear speed and density of the gas increase. As a result the direct losses of the catalyst (per run) are substantially higher than at atmospheric pressure, which leads to a substantially shorter service life of the packet of nettings. Measurements of the distribution of speeds in contact apparatuses show that substantial heterogeneities of the flow exist over the cross section of the apparatus. In spite of the use of distribution devices of various constructions, the ammonia mixture may pass through one third of the area of the catalyst at a speed that is almost three times greater than the speed through the remaining two thirds of the area. A change in the hydrodynamic situation in the converter has a substantial influence on the value of the loss of platinoids.
A layer of regular structure, formed by a honeycomb catalyst block and positioned directly after the layer of platinoid nettings, not only makes it possible to reduce the amount of platinoids by reducing the number of nettings in the first stage, but also to reduce losses in the process of the run of nettings while maintaining the productivity of the product [RF patent No. 2100068, IPC 6 B 01 J 23/78, BI No. 36, 1997].
The solution most similar to the claimed technical solution is a method for catalytic oxidation of ammonia, the method consisting of passing a reaction gaseous mixture comprising ammonia and oxygen through a two-stage catalytic system in which the first stage in the downstream direction of the flow of gas is a layer of platinoid nettings, the second stage is one layer of a honeycomb catalyst of regular structure, and the ratio of the average working speed to the speed of sound under these conditions is maintained within the range of from 4.8xc2x710xe2x88x924 to 0.24 in the jets of the gaseous mixture, moving along the honeycomb channels of the catalyst [RF application No. 97118457/25, IPC 6 C 01 B 21/26, BI No. 21, 1998].
During oxidation of the ammonia in both the first stage (platinoid nettings) and in the second stage (catalyst block), the process proceeds in an external diffusion mode. The effectiveness of the oxidation process is determined by the rate of mass exchange between the surface of the catalyst (the platinoid netting, the catalyst block) and the gaseous phase. In accordance with the mechanisms for the laminar flow of gas in the channels of a catalyst block, the coefficients of mass exchange are constant along the length of the channel everywhere except at the input part of the block. Due to the transient character of the flow of gas at the input into the catalyst block, the coefficients of mass exchange at these sections is 3-5 times higher than the average value along the length of the block. Thus, if 2 blocks 50 mm long are set up instead of one block 100 mm long, this will result in an increase in the degree of conversion of the ammonia. Simultaneously, due to the transient character of the flow of gas at the input sections of the channels in the catalyst block, the hydraulic resistance of the system as a whole will increase. This results in enhancement of the homogeneity of the hydrodynamic situation in the layer of the catalyst. Thus, a reduction of the initial heterogeneities of the speed field, which are characteristic for this construction of a reactor, will take place. As a result, the local heterogeneities of speed will be reduced at separate sections of the platinoid netting and the loss of platinoids will be substantially reduced.
The object of the instant invention is to enhance the yield of the desired product, for example, in the process of the production of nitric acid and hydroxylamine sulfate, this is nitric oxide, in the process of the production of prussic acidxe2x80x94HCN.
The object is attained by passing a reaction gaseous mixture comprising ammonia and an oxygen-containing gas through a two-stage catalytic system in which the first stage in the downstream direction of the flow of gas is a layer of platinoid nettings, and 2-5 spaced layers of a catalyst of a regular honeycomb structure are used in the second stage of the catalytic system. Each separate layer of the catalyst of honeycomb structure is arranged at a distance less than 60 mm from the neighboring layer, mainly at a distance which is in the range of from one half the thickness of the wall of the block channel to the thickness of that wall, a gas-permeable inert material is arranged between the layers of the catalyst of honeycomb structure. The height of each layer of the catalyst of honeycomb structure is not more than 0.5 Rexc2x7de, where Re is the Reynolds number equal to from 1 to 5xc2x7104, de is the hydraulic diameter of a catalyst channel equal to from 1 to 20 mm.
The ratio of the value of the hydraulic resistance of the second stage of the catalytic system to the value of the hydraulic resistance of the first stage is from 0.2 to 4.
A catalyst of honeycomb structure is used in the process, the porosity of which, characterizing the volume of the empty spaces, or the open surface of which is 0.1-0.6, the thickness of the wall of which is (0.1-1.0)de, here de is the hydraulic diameter of a channel of the catalyst, equal to 1-20 mm.
The catalyst of honeycomb structure comprises oxides of base metals in its composition and is a mixture of oxides of the general formula (AxByO3z)k(MemOn)f, where: A is a cation of Ca, Sr, Ba, Mg, Be, Ln or mixtures thereof; B is cations of Mn, Fe, Ni, Co, Cr, Cu, V or mixtures thereof, x=0-2, y=1-2, z=0.8-1.7; MemOn is aluminum oxide and/or silicon oxide, zirconium oxide, chromium oxide, aluminosilicates, oxides of rare-earth elements (REE) or mixtures thereof, m=1-3, n 1-2, k and f are % by weight, with a ratio k/f=0.01-1, % by weight: ferric oxide 70-94, aluminum oxide 1-29, silicon oxide and/or REE oxides, zirconium oxide 1-29, the specific surface of which is more than 5 m2/g.